42 Harrington et al.: Invasive and the Green Industry

INVASIVE PLANTS AND THE GREEN INDUSTRY By Robin A. Harrington1, Ronald Kujawski2, and H. Dennis P. Ryan3

Abstract. There are many motivations for introducing offices were recommending the planting of introduced to areas outside their native range. Non-native species, such as autumn olive (Elaegnus umbellata) and plants can provide food, medicine, shelter, and ecosystem multiflora rose (Rosa multiflora), for wildlife food and cover. services, as well as aesthetic value. However, some species, Despite the many benefits provided by introduced species, such as Norway maple (Acer platanoides), Japanese barberry there has been growing concern recently among land ( thunbergii), and Oriental bittersweet ( managers regarding the negative impacts when introduced orbiculatus), have escaped from cultivation, with severe species escape cultivation and become invasive in local ecological and economic consequences. The approval of a habitats. Invasive plant species pose a major threat to National Management Plan in June 2001 biodiversity, habitat quality, and ecosystem function. Invasive has major implications for future plant introductions and plants can also have devastating economic impacts, through has generated concern in the green industry. There is loss of revenue (reduced agricultural and silvicultural general agreement among plant professionals regarding (1) production in the presence of invasive species) and the high a need for education of industry people and client groups costs associated with control programs. The magnitude of on the issue of invasive species, (2) minimizing economic these impacts resulted in Executive Order 13112, issued by disruption to the nursery industry, and (3) requirements for President Clinton on February 3, 1999, directing all federal objective data to support listing of species as invasive. agencies to prevent and control introductions of invasive Nursery and landscape associations in several states have species. On January 18, 2001, the National Invasive Species already taken steps toward listing invasive species in Council approved a National Invasive Species Management cooperation with conservation organizations. This proactive Plan (www.invasivespecies.gov/council/nmp.shtml)), which has approach ensures industry representation and input in major implications for the future introduction of plant species developing policy and, when coupled with intense and that will in turn impact the horticulture industry. ongoing educational programs for industry people and their Our objectives are to (1) present an overview of the clients, could dramatically reduce the introduction and concerns that have led to the National Invasive Species spread of invasive species. Management Plan, (2) present examples of the impacts of Key Words. Invasive plants; Norway maple; Japanese three important nursery species when they invade natural barberry; Oriental bittersweet; Acer platanoides; Berberis and managed areas, and (3) propose strategies to address thunbergii; . the concerns of the green industry regarding constraints on future plant introductions and distribution.

WHAT IS AN INVASIVE? For centuries, there have been many motivations for human In any discussion of the negative impacts of invasive plant dispersal of plant species around the globe. We have long species, it is important to put the invasive species issue into recognized the importance of non-native plants in providing context. First of all, not all introduced species become invasive. food, medicine, shelter, and ecosystem services, as well as In fact, only a small proportion do. Williamson and Fitter (1996) cultural enjoyment (Ewel et al. 1999; Mack 2001). Introducing proposed the “Tens Rule” based on statistical analysis of a new plants has been a common cultural phenomenon since the number of British animals and plants. The rule states that, on beginning of time. In North America, the Spanish were the first average, one in ten species introduced (either intentionally or Europeans to introduce a new tree to the Americas, the peach. accidentally) will escape from cultivation. Only one in ten of The northern Europeans brought with them other food trees these escaped species will become naturalized and form self- such as apple, pear, and cherry. It wasn’t until the1860s that sustaining populations. In turn, only one in ten of these natural- serious effort went into collecting new plants for America. ized species will likely become invasive. However, because of the Some of the famous collectors of this period were Dr. George vast number of introductions over the years, even the relatively Hall, who worked for 15 years in China; Dr. Charles Sargent of small percentage of the total that become invasive has resulted in the Arnold Arboretum; and Professor William Clark of the a great number of invasive plant species. Massachusetts Agricultural College, who collected plants while One of the major exceptions to the Tens Rule, where the working with the Japanese to establish an agricultural college in chance of moving from one stage in the invasion process to Sapporo during 1876. Until recently, state and federal wildlife the next is much greater than 10%, includes plants intro- Journal of Arboriculture 29(1): January 2003 43

duced intentionally for cultivation. This fact is not surprising other intact forests (Webb and Kaunzinger 1993) where it because these species are selected to do well in the region reduces understory biodiversity (Wyckoff and Webb 1996). where they are introduced. Also, characteristics that make Norway maple has been overused as a street tree because it species desirable as street trees or ornamental plantings also was readily available when Dutch elm disease eliminated most increase the likelihood that they will become invasive. Urban American elms (Ulmus americana) as our principal city tree. and suburban sites typically require plants that will be able to Norway maple was and is inexpensive and, when planted in survive in harsh conditions. In many cases, the plants are put tough urban conditions it did well, resulting in attractive, tree- into a poor soil with low oxygen conditions. They are exposed lined streets. to rapid changes in air temperature, and the light conditions can be poor. A tree that can live in these conditions is a Japanese Barberry survivor and can live just about anywhere. Japanese barberry () is a thorny, perennial However, the success of an invasion is affected not only by shrub native to central and southern Japan (Ohwi 1965). It was the characteristics of the invader but also by the characteristics first planted in North America in the late 1800s. By the 1920s, of the site being invaded. There has been growing interest in the species was naturalized in New England and, by the middle developing the ability to predict which sites are most susceptible of the century, it was considered a pest (Silander and Klepeis to invasion. There is general agreement among managers and 1999). The primary reason it was planted, and continues to be scientists that invasion is facilitated by disturbance. Disturbance planted, is its attractive foliage and (Figure 1). In addition, increases plant invasions by providing suitable microsites for it is often planted as a hedge because its thorny branches and germination and seedling establishment and by increasing light bushy habit make an effective barrier (Ehrenfeld 1997). and nutrient availability that enhance seedling survival and growth (Orians 1986; Hobbs and Huenneke 1992). Invasive species are better able to take advantage of high resource availability than many native species. For example, germination and seedling survival of two invasive woody species, paperbark (Melaleuca quinquinerva) and Brazilian pepper (Schinus teribinthifolius), were greater in disturbed communities than in intact native communities (Ewel 1986). Opening up of forest canopies through management activities or natural disturbance increases light availability and often favors growth and survival of invasive species (Horvitz et al. 1998; Knapp 1998). Although natural disturbance has sometimes been implicated in the establishment of invasive species in natural areas (Horvitz et al. 1998), our increasing fragmentation of the landscape, especially at the suburban–rural interface, is responsible for the establish- ment and spread of a great number of invasive species. Recent observations of successful invasions in intact forest ecosystems indicate that although disturbance facilitates invasion, it is not a prerequisite. Certain shade-tolerant species have been able to establish in forest understories with dire consequences for silvicultural and natural area management. We will present three examples across a range of growth forms: a tree, a shrub, and a vine. We will describe the characteristics that make these invasive species desirable as street trees and ornamental plantings, which of these characteristics increase their invasiveness, and the consequences of their invasion in forest habitats.

Norway Maple Norway maple (Acer platanoides) is a commonly planted street tree in the United States because of its ability to tolerate stressful urban environments, combined with its ease of propagation and a variety of desirable Figure 1. Attractive foliage and berries of Japanese (Nowak and Rowntree 1990). From this widespread barberry make this species a very desirable ornamental planting, Norway maple has become naturalized and has (photo taken by Les Mehrhoff, University of Connecticut). readily spread into urban woodlands (Bertin et al. 2001) and 44 Harrington et al.: Invasive Plants and the Green Industry

Several characteristics of barberry physiology and that it will continue to be planted and spread across the anatomy facilitate the spread of the species and cause it to landscape. Oriental bittersweet is considered a pest by many form dense thickets in some areas. Barberry can form dense land managers because of its ability to readily overtop native stands because of the high initiation rate of stems grown trees and shrubs along roadsides, in clearings, and in forest from existing root collars, clonal spread by stems that form gaps (Dreyer et al. 1987; McNab and Meeker 1987) (Figure 3). adventitious roots, and high rates of seedling recruitment In a recent study, this vine species grew an average of 4.5 m (Ehrenfeld 1999). Barberry foliage is unpalatable to deer, so in length over 12 weeks in partial shade, with certain there is little mortality due to browsing (Ehrenfeld 1997). individuals growing up to 9 m over the same interval The berries are eaten and the spread by a variety of (Ellsworth 2002). Bittersweet seeds are able to germinate in animal species, including white-tail deer, turkeys, and grouse a variety of light conditions, including partial and dense (Ehrenfeld 1997). Barberry can survive and grow in a broad shade (Patterson 1974). The species is also very adaptable; range of light and moisture conditions. However, barberry is in one experiment, shade-grown plants altered their of particular concern because it is able to invade intact photosynthetic light response to nearly that of sun-grown forest understory habitats where it forms impenetrable plants in 8 days (Patterson 1975). This ability could explain thickets and prevents natural forest regeneration (Figure 2). its rapid domination of gaps following disturbances such as Barberry is extremely shade tolerant; it can survive in as windthrow, timber harvesting, and road construction. Rapid little as 1% full sun and can produce berries in conditions of growth over stumps, saplings, and adult trees leads to only 4% full sun (Silander and Klepeis 1999). However, this silvicultural problems by suppressing natural regeneration species also responds readily to increases in light and and planted stock through shading, girdling, and stem nitrogen availability (Cassidy 2002). deformity, and the added weight increases the possibility of ice damage (McNab and Meeker 1987). In addition to effects on individual tree growth and survival, it is possible that growing from tree to tree increases the likelihood of multiple trees falling together as a result of logging or windthrow (Putz 1991).

CONCERN OF GREEN INDUSTRY The green industry may feel unfairly targeted while trying to satisfy the public demand for non-native species. However, 85% of woody invasive species in North America were introduced by the landscape industry (Reichard and Hamilton 1997); therefore, dialog between landscape industry and natural area managers needs to be established. Some state nursery and landscape associations are actively addressing the issue. For example, in Massachusetts, representatives of the Massachu- setts Nursery and Landscape Association (MNLA) sit on the Massachusetts Invasive Plant Evaluation Figure 2. The combination of multiple arching stems produced from each root collar and the ability of these stems to form Committee, which is currently evaluating criteria for adventitious roots when they lean over and touch the soil allows determining invasive plants and developing a list of Japanese barberry to form impenetrable thickets (photo taken by invasives based on that criteria. The executive Robin Harrington). director of MNLA co-chairs that committee. To stimulate discussion among its members, the Virginia Nursery and Landscape Association Oriental Bittersweet (VNLA) through its newsletter (Bruce 2001) has published a Oriental bittersweet (Celastrus orbiculatus) is a , list of the 11 plants that the Virginia Native Plant Society has woody vine that was introduced as an ornamental from identified as most troublesome (Table 1). The intended eastern Asia in 1860. It has become naturalized in the outcome of publishing this list is to develop an agreement by northeastern United States and is now present in every VNLA that would remove these plants from production and county in Massachusetts (Sorrie and Somers 1999). Bitter- commercial trade. sweet produces abundant fruits that are readily dispersed On the national level, the American Nursery and by birds. The colorful fruits also make bittersweet a very Landscape Association (ANLA) has developed a “Voluntary desirable ornamental, which may increase the likelihood Code of Conduct for Nursery Professionals” to guide the Journal of Arboriculture 29(1): January 2003 45

ment, academia, ecology, and conservation organizations on specific invasive species, phase out existing stocks of those species in regions where they are considered a threat.” American Nurseryman, a trade magazine, conducted a roundtable discussion (American Nurseryman 1999) with nine plant professionals to gather their views on the invasive plant issue. Several common themes were expressed by the participants. Themes included a need for education of industry people and client groups on the issue of invasives; minimizing economic disruption to the nursery industry; and requirement for objective data to support listing of species as invasive. The primary concerns expressed by the green industry center on a perceived broad-brush approach to the invasive issue. There is fear that national mandates prohibiting the sale or use of certain plant species in all regions of the country would be unnecessarily restrictive in some cases. Due to regional climate and environmental differences, species that may be invasive in one region would not be invasive in others, yet federal regulations would prohibit their use anywhere. There is a preference for regional solutions or mandates to coping with some identified invasive species. Another concern is the exclusion of industry representa- tion on and input in bodies that are involved in developing policy. The industry feels strongly that they must be included in any decision making that affects their business. The preference of the industry (and many nonindustry groups, for that matter) is that voluntary standards be established for the industry, coupled with intense and ongoing educa- tional programs for industry people and their clients. Figure 3. Oriental bittersweet can readily climb and The emphasis on a “natives only” alternative to invasive overtop trees along roadsides (photo taken by Robin plants by some is also a concern to the industry. While Harrington). green-industry people grow, sell, and plant native species, the bulk of plant materials in their inventory are made up of Table 1. Virginia Native Plant Society list of invasive non-native species, the vast majority of which are not plants carried by the Virginia nursery industry (Bruce identified in any way as being invasive. Industry people 2001). would prefer that the invasive plant issue and native plant issue be kept separate. Tree-of-heaven Ailanthus altissima Akebia, chocolate vine Akebia quinata Porcelain Ampelopsis brevipedunculata COMMON GROUND Oriental bittersweet Celastrus orbiculatus In the past, invasive species have been used extensively Autumn olive Elaeagnus umbellata throughout urban-to-rural gradients across the landscape. Weeping love grass Eragrostis curvula However, based on current knowledge, it’s clear that we Silver lace, fleece vine Polygonum aubertii need to limit the use of identified invasive plants. Some Kudzu Pueraria montana invasive plant species should be banned completely because European eater-milfoil Myriophyllum spicata they are so destructive and costly to manage. Some other Hydrilla Hydrilla verticillata plants should be limited to urban areas. Some groups want Parrot’s feather Myriophyllum aquaticum to limit the green industry to using only native plants and, although the authors support the use of native species in actions of plant professionals in dealing with invasive plants. general, we feel this practice could actually increase the Among the provisions in the code (http://www.mobot.org/ management of plant problems in some cases. As an iss/nurserycode2.htm) is the following: “Where agreement example, we have available American sycamore (Platanus has been reached among nursery associations and govern- occidentalis) and London planetree (Platanus acerifolia) that 46 Harrington et al.: Invasive Plants and the Green Industry

can be specified as park or street trees. They are similar in Bertin, R.I., T. Cantwell, and B. Berstene. 2001. Introduced appearance and will grow as street trees. While American Tree Species in Urban Woodlands in Worcester, sycamore is frequently infected with anthracnose, London Massachusetts. Poster given at the 86th Annual Meeting planetree is not seriously infected with the disease and, of the Ecological Society of America, 5–9 August 2001, importantly, is not invasive. Madison, WI, Along with the need to understand and mitigate the Cassidy, T.M. 2002. Effects of soil acidity and resource impacts of invasive plant species already in the landscape, availability on the growth of Japanese barberry (Berberis we need to stop the introduction of species known to be thunbergii). M.S. Thesis, Department of Natural Resources invasive and to prevent the introduction of new species that Conservation, University of Massachusetts, Amherst, MA. may have the potential to be invasive. Several methods for Dreyer, G.D., L.M. Baird, and C. Fickler. 1987. predicting which species could become invasive have been and Celastrus orbiculatus: Comparisons of reproductive proposed (Mack 1996). One approach relies on identifying potential between a native and an introduced woody vine. key ecological and life history characteristics common to Bull. Torrey Bot. Club 114:260–264. many invasive plant species (Newsome and Noble 1986; Ehrenfeld, J.G. 1997. Invasion of deciduous forest preserves Rejmánek and Richardson 1996), but it is also important to in the New York metropolitan region by Japanese barberry look at current geographic distributions of species. Plants (Berberis thunbergii DC.). J. Torrey Bot. Soc. 124:210–215. with a wide geographic native range are more likely to ——. 1999. Structure and dynamics of populations of become invasive when they are introduced elsewhere Japanese barberry (Berberis thunbergii DC.) in deciduous because of their ability to tolerate a wide range of condi- forests of New Jersey. Biol. Invas. 1:203–213. tions (Goodwin et al. 1999). Also, species that have proven Ellsworth, J.W. 2002. Controls on the establishment and early to be invasive elsewhere are more likely to become invasive growth of Oriental bittersweet (Celastrus orbiculata following new introductions (Reichard and Hamilton 1997). Thunb.). M.S. Thesis, Department of Natural Resources The ideal approach would result in rejection of all poten- Conservation, University of Massachusetts, Amherst, MA. tially invasive species while minimizing the rejection of Ewel, J.J. 1986. Invasibility: Lessons from South Florida, pp noninvasive species. The most promising approaches 214–230. In Mooney, H.A., and J.A. Drake (Eds.). incorporate the biological and ecological characteristics of Ecology of Biological Invasions of North America and the species, biogeographical information, and status of the Hawaii. Springer-Verlag, New York, NY. species as a weed elsewhere (Reichard and Hamilton 1997; Ewel, J.J., D.J. O’Dowd, J. Bergelson, C.C. Daehler, C.M. Pheloung et al. 1999). D’Antonio, L.D. Gomez, D.R. Gordon, R.J. Hobbs, A. Holt, K.R. Hopper, C.E. Hughes, M. LaHart, R.R.B. Leakey, CONCLUSIONS W.G. Lee, L.L. Loope, D.H. Lorence, S.M. Louda, A.E. Education will be the key to reducing the problems associ- Lugo, P.B. McEvoy, D.M. Richardson, and P.M. Vitousek. ated with invasive plants. The green industry and environ- 1999. Deliberate introductions of species: Research mental groups need to finish and publish their lists of needs. BioScience 49:619–630. problem plants. While state-by-state lists could be pro- Goodwin, B.J., A.J. McAllister, and L. Fahrig. 1999. duced, regional lists would be better. Landscape architects, Predicting invasiveness of plant species based on tree wardens, and other professionals involved with the biological information. Conserv. Biol. 13:422–426. writing of planting specifications need to adopt the new Hobbs, R.J., and L.F. Huenneke. 1992. Disturbance, diversity, plant guidelines. To accomplish this goal, we will have to and invasion: Implications for conservation. Conserv. educate the general public that some of their favorite plants Biol. 6:324–337. will no longer be available. We will need to replace some of Horvitz, C.C., J.B. Pascarella, S. McMann, A. Freedman, and these popular plants with sterile hybrids or other plant R.H. Hofstetter. 1998. Functional roles of invasive non- species. indigenous plants in hurricane-affected subtropical Lastly, the green industry needs to take a proactive role hardwood forests. Ecol. Appl. 8:947–974. by working with environmental groups in identifying the Knapp, L.B. 1998. Growth of Ailanthus altissima in Old problems. By working to reduce an increasingly serious Growth Forest Gaps. M.S. Thesis, Bard College, situation, the green industry will assist in ensuring that our Annandale-on-Hudson, NY. 74 pp. natural areas remain natural. Mack, R.N. 1996. Predicting the identity and fate of plant invaders: Emergent and emerging approaches. Biol. LITERATURE CITED Conserv. 78:107–121. American Nurseryman. 1999. Invasives roundtable. Am. ——. 2001. Motivations and consequences of the human Nurseryman 190(2):54–77. dispersal of plants, pp 23–34. In McNeely, J.A. (Ed.). The Bruce, J. 2001. News—Invasive plants? Let us know! Va. Great Reshuffling: Human Dimensions of Invasive Alien Nursery Landscape Assoc. Newsl. 71(1):17. Species. ICUN, Gland, Switzerland, and Cambridge, UK. Journal of Arboriculture 29(1): January 2003 47

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Es gibt ein generelles Einverständnis unter der también valor estético. Sin embargo, algunas especies, tales Professionellen 1. bezüglich eines Bildungsbedarfs in der como el maple Norway y el cerezo Japonés, han escapado Industrie und der an Importen interessierten Klientel, 2. im del cultivo, con severas consecuencias ecológicas y Minimieren einer ökonomischen Unterbrechung in der económicas. El Plan Nacional de Manejo de Especies produzierenden Baumschulindustrie und 3. in den Invasoras aprobado en Junio de 2001 tiene implicaciones Anforderungen für objektive Daten und Fakten, um die para la introducción de plantas en el futuro y ha generado betroffenen Pflanzen überhaupt zu klassifizieren und zu preocupación en la industria verde. Existe un acuerdo listen. Baumschul- und Landschaftsbauverbände in general entre los profesionales de las plantas en cuanto a (i) verschiedenen Bundesstaaten haben bereits Schritte una necesidad de educación a la gente de la industria y unternommen, um die Listung invasiver Arten in grupos de clientes en el tema de las especies invasoras, (ii) Kooperation mit den landschaftserhaltenden minimizar el impacto económico en la industria de los Organisationen zu erreichen. Dieser proaktive Ansatz viveros, y (iii) requerimiento de datos objetivos de soporte sichert Industriepräsentation und einen Input in der de listas de especies invasoras. Los viveros y las asociaciones Entwicklungspolitik und könnte, wenn er verbunden wird paisajísticas en varios estados han tomado iniciativas en el mit intensiven und fortlaufenden Ausbildungsprogrammen sentido de elaborar las listas en cooperación con für Industriebeschäftigte und ihre Klientel die Einführung organización de conservación. Esta aproximación asegura und Verbreitung invasiver Pflanzen drastisch reduzieren. representación a la industria en el desarrollo de políticas, y Resumen. Existen muchas motivaciones para la cuando se trabaja en programas educativos para la gente de introducción de especies de plantas a áreas fuera de su la industria y los clientes, se podría reducir dramáticamente rango nativo. Las plantas no-nativas pueden proporcionar la introducción y dispersión de especies de plantas alimento, medicina, abrigo, y servicios al ecosistema, como invasoras.